Teratology is the study of abnormalities in fetal development and birth defects. Some key causes of birth defects include certain medications (such as thalidomide, diethylstilbestrol, valproic acid), infections (rubella, toxoplasmosis), and environmental factors (alcohol). There are critical periods in development when the fetus is most susceptible to teratogens. Understanding how teratogens act can help prevent birth defects and develop new safe drugs for pregnant women.